The invention relates to a polypeptide (8F4 molecule) having the biological activity of costimulating T cells. The invention further relates to monoclonal antibodies against the 8F4 molecule and hybridoma cells which produce the monoclonal antibodies. The invention additionally relates to the use of substances which inhibit the biological activity of the polypeptide 8F4 according to the invention, in particular monoclonal antibodies, natural or synthetic ligands, agonists or antagonists, as pharmaceuticals. In particular, the invention relates to the use of these substances for the prevention or therapy of disorders in which the immune system is involved, in particular for the treatment of autoimmune diseases and for the prevention of rejection reactions with organ transplants. The invention additionally relates to the use of the 8F4 molecule or of cells which contain the 8F4 molecule as pharmaceuticals, in particular for the prevention or therapy of disorders in which the immune system is involved, in particular for the treatment of cancers, Aids, asthmatic disorders or chronic viral diseases such as HCV or HBV infections. The invention further relates to the use of substances which specifically recognize the polypeptide according to the invention, in particular monoclonal antibodies, natural or synthetic ligands, agonists or antagonists, for the diagnosis of disorders in which the immune system is involved. In particular, the invention relates to diagnosis by means of an ELISA detection, a flow cytometry or a Western blot, a radioimmunological detection, a nephelometry or a histochemical staining.
T lymphocytes recognize their antigen, which is presented by “antigen-presenting cells”, for example dendritic cells, B cells and macrophages, through their T-cell receptor. Recognition of the antigen by the T-cell receptor alone is, however, in most cases insufficient for adequate activation of T lymphocytes. The latter makes additional simultaneous stimulation (also called “costimulation” hereinafter) by other receptor molecules on the surface of the T lymphocytes necessary. One of these receptor molecules is the so-called CD28 receptor which is stimulated by the costimulating molecule B7. If these “costimulatory” molecules, for example CD28, are effective, then the activation of the T cell reaches an adequate level after recognition of the antigen by the T-cell receptor. After such a complete activation, the T cell expresses additional molecules, for example CD25, CD69, CD71, on the surface and synthesizes numerous cytokines, for example IL-2 and IFN-γ, which function as messengers. Both these additional surface molecules and the cytokines serve for the T cell to exchange information with other cells in the immune system. The activated T cells direct the entire antigen-specific immune defences through the additional surface molecules and the cytokines. Both the generation of cytotoxic cells (“killer cells”) and the generation of antigen-specific antibodies by B cells is controlled in this way. Cytotoxic cells, as well as the specifically formed antibodies, eliminate viral or bacterial pathogens which enter the body. In some cases, however, the immune response goes too far, and the immune system is directed against the body's own cells. This leads to the occurrence of “autoimmune diseases”, for example to rheumatoid arthritis, ankylosing spondylitis, Sjögren's syndrome, ulcerative colitis inter alia. One of the essential sites of cooperation between antigen-activated T cells and other cells of the immune system are the secondary lymphatic organs, including the tonsils. This is where the T lymphocytes are activated by the antigen presented by dendritic cells, and this is where T lymphocytes interact with B cells. Through this interaction, B cells secrete, after several intermediate stages of differentiation, antigen-specific antibodies of the IgM and IgG types.
The costimulatory molecule which has been characterized best and is among the most effective to date is the CD28 surface molecule (called CD28 receptor or CD28 hereinafter) which is constitutively expressed on a large fraction of T cells. Costimulation by CD28 in vitro leads, after recognition of the antigen by the T-cell receptor, to a very large increase in cytokine secretion, for example of IL-2 and IFN-γ, and to a marked up-regulation of the expression of cell surface molecules such as CD25, CD69, CD71, which are necessary for interaction of T cells with other immune cells, for example B lymphocytes; cf. Chambers and Allison, Current Opinion in Immunology 9 (1997), 396–404. Costimulation via the CD28 receptor can also markedly increase the proliferation of T lymphocytes. In addition, costimulation via the CD28 receptor optimizes the T-cell control of B-lymphocyte function so that there is increased secretion of antibodies.
If the function of the CD28 receptor is abolished, there is a drastic loss of function in the immune defences. This has been shown by means of a transgenic mouse in which the CD28 gene was destroyed by homologous recombination (a so-called “CD28 knock-out”). The destruction in this way of activation of the antigen-specific T cells leads to lack of costimulation. This in turn leads to a disturbance of T-cell function, that is to say to a reduced proliferation of T cells and to a drastically reduced synthesis of various cytokines. The lack of costimulation eventually leads to a reduced function of the antigen-specific immune defences. Thus, inter alia, the formation of antigen-specific IgG1 and IgG2 antibodies by B lymphocytes is reduced to 10% of the normal level through the lack of CD28; cf. Shahinian et al., Science 262 (1993), 609–612; Lucas et al. Journal of Immunology 154 (1995), 5757–5768. It is also possible in vitro to prevent the Aids virus entering T lymphocytes by costimulation by CD28; cf. Riley et al., Journal of Immunology 158 (1997), 5545–5553. Corresponding experiments in vivo have not yet been carried out. It is known that CD28 switches on many cytokine genes which may lead to considerable side effects in vivo. Blockade of CD28 receptors by a soluble CTLA-4 immunoglobulin molecule has been employed successfully in a monkey model to prevent the rejection of transplanted kidneys. In this case, CTLA-4 had been employed in combination with an antibody against the CD40 ligand molecule; cf. Kirk et al., Proc. Natl. Acad. Sci. USA 94 (1997) 8789–8794. However, blockade of CD28 receptors affects all T lymphocytes and not just those already activated because CD28 is constitutively expressed on T lymphocytes.